Cavitation within a stream of liquid occurs when the fluid pressure of the liquid drops below its vapor pressure in a controlled flow stream of fluid, such as in a pipe or control valve, and gas bubbles are formed in the flow stream. Subsequently, when the fluid pressure recovers to a level above the vapor pressure, the gas bubbles collapse and implode violently in a process that produces a significant high energy acoustic wave. Sometimes, the formation of the initial gas bubbles is referred to as “flashing,” whereas the implosion of the gas bubbles is referred to as “cavitation.” For purposes of this description, however, the term “cavitation” is hereafter used to encompass the overall process of both the formation and the implosion of the gas bubbles unless clearly indicated otherwise.
Control valves often have at least one region of reduced flow area somewhere between an inlet into the valve body and an outlet from the valve body. One typical region of reduced flow area is at or near the orifice defined by the valve seat and/or proximate the valve trim. Therefore, fluid flowing through a control valve usually experiences some level of pressure drop or pressure loss as it travels through the reduced flow area. The pressure will typically have a lowest value somewhere inside or immediately downstream of the control valve body before increasing somewhat. In some circumstances, these lower pressure conditions can cause cavitation in the control valve between the valve trim and the outlet and/or in the pipe immediately adjacent the outlet.
Cavitation within the stream of liquid passing through the control valve can be problematic. Cavitation inside or near the physical boundaries of the control valve can cause severe physical damage to the control valve or the adjacent piping components. For example, cavitation at or near the inner wall surface of the flow channel through the valve body or the valve trim may cause damage to the pressure boundary, the valve trim, or other valve components. The damage typically accumulates over time such that periodic maintenance must be performed on the control valve to repair damage to components caused by the cavitation. When scheduling maintenance on many industrial process lines, it is desirable to be able to accurately predict when a particular valve or other piece of equipment will require repair, up to and including replacement, before the process line is shut down and opened up.